The present invention relates to diagnostic and therapeutic radiology equipment and, more particularly, to improved cathode assemblies used in x-ray generating equipment, such as computerized axial tomography (C.A.T.) scanners. Most particularly, the invention is directed to improved x-ray tube cathode cup assemblies having a solid, one piece insulator unit.
Each x-ray tube is normally enclosed in an oil-filled protective casing. A glass envelope contains a cathode plate, a rotating disk target and a rotor that is part of a motor assembly that spins the target. A stator is provided outside the tube proximate to the rotor and overlapping therewith about two-thirds of the rotor length. The glass envelope is enclosed in an oil-filled lead casing having a window for the x-rays that are generated to escape the tube. The casing in some x-ray tubes may include an expansion vessel, such as a bellows.
X-rays are produced when, in a vacuum, electrons are released, accelerated and then abruptly stopped. This takes place in the x-ray tube. To release electrons, the filament in the tube is heated to incandescence (white heat) by passing an electric current through it. The electrons are accelerated by a high voltage (ranging from about ten thousand to in excess of hundreds of thousands of volts) between the anode (positive) and the cathode (negative) and impinge on the anode, whereby they are abruptly slowed down. The anode, usually referred to as the target, is often of the rotating disc type, so that the electron beam is constantly striking a different point on the anode perimeter. The x-ray tube itself is made of glass, but is enclosed in a protective casing that is filled with oil to absorb the heat produced. High voltages for operating the tube are supplied by a transformer. The alternating current is rectified by means of rectifier tubes (or "valves") in some cases by means of barrier-layered rectifiers.
For therapeutic purposes--e.g., the treatment of tumors, etc.--the x-rays employed are in some cases generated at much higher voltages (over 4,000,000 volts). Also, the rays emitted by radium and artificial radiotropics, as well as electrons, neutrons and other high speed particles (for instance produced by a betatron), are used in radio therapy.
X-ray tube performance can be affected by the alignment of the filament in the cathode assembly. Specifically, during x-ray tube manufacturing, it is important to be able to initially align the filament and have it stay aligned during completion of the manufacturing cycle and during operation of the x-ray tube.
Previously, coiled tungsten filaments used in x-ray tubes were assembled and then aligned in the cathode cup. Once assembled, the filaments were heated to about 2800.degree. C. to produce the desired microstructure. During this heating, when assembled in the cathode cup, many filaments sagged and thus move out of alignment making it necessary to reseat them in the cathode cup and repeat the flashing or the heating to 2800.degree. C. In some instances, this step had to be repeated up to as many as five (5) times until the filament alignment in the cathode cup was obtained with the desired microstructure.
A prior method of assembling the filament included an operator determining from a pattern the proper shape of each insulating flange so that each individual prior are insulator would fit into the cathode cup in accordance with the number of filaments to be positioned in the cathode cup. During the flange cutting, burs were sometimes formed on the flange edge. When the flange was connected to the cathode cup surface, such as by welding, the insulator was twisted out of proper alignment. Thus, after assembly of the filament in the twisted, misaligned insulator, the filament was misaligned sufficiently to cause x-ray failures.
Because of undetected insulator misalignment and the resulting misalignment of the filament(s), the need for a new insulator which reduced or eliminated filament alignment in the x-ray tube cathode cup became apparent. Thus, there is a need for an insulator member, preferably one piece, made of, for example, a ceramic material that is sized to slide into the cathode cup in a fixed position to provide proper filament alignment and thus at least reduce or eliminate failures related to filament misalignment due to insulator misalignment.